The term mirror world was first introduced by David Gelertner in his book "Mirror Worlds Or the Day Software Puts the Universe in a Shoebox How It Will Happen and What It Will Mean" published in 1991. He describes a world where ""You will look into a computer screen and see reality," he writes in his prologue. "Some part of your world -- the town you live in, the company you work for, your school system, the city hospital -- will hang there in a sharp color image, abstract but recognizable, moving subtly in a thousand places. This mirror world you are looking at is fed by a steady rush of new data pouring in through cables. It is infiltrated by your own software creatures, doing your business." (as quoted in the NYT)

The metaverse has come to encompass all that Gelertner presciently described as a mirror world. Now, however, the term mirror world persists as a more narrow imagining: it is used to describe data that is more tightly tied to the real. In some definitions the mirror world's function is bound to earth's geography. In the 1995 Metaverse Roadmap locates Mirror Worlds in the quadrant defined by external information and realistic simulation. "Mirror worlds are informationally-enhanced virtual models or “reflections” of the physical world. Their construction involves sophisticated virtual mapping, modeling, and annotation tools, geospatial and other sensors, and location-aware and other lifelogging (history recording) technologies." (source)  

Writing in 2022, the author Promise defines a mirror world as "a one-to-one map with a scope that is virtually impossible to completely exhaust. A digital reflection of the physical environment, it makes an effort to map real-world structures in a manner that is accurate from a geographical standpoint. When it is finished, our material world and the digital realities will be practically indistinguishable from one another."  

Alternatively, the term mirror world has been expanded to describe a relationship between digital and analog objects that extends beyond facsimilie: 

"Mirroring occurs when physical things, which can be perceived and acted upon by humans in the physical world, have a digital counterpart (or extension) in the mirror world, so that they can be observed and acted upon by agents. Vice versa, an entity in the mirror world that can be perceived and acted upon by software agents can have a physical appearance (or extension) in the physical world—for example, through AR—so that it can be observed and acted upon by humans." (Croatti and Ricci).